Study on the soft mode around the phase transition tenperature of ferroelectric crystals through low-frequency dielectric spectrum measurements.

研制了低频介质谱测量系统。该测量系统能够在45 MHz ~ 15 GHz的频率范围内，在80 K ~ 450 K的温度范围内快速提供可靠的测量数据。利用研制的测量系统，研究了铁电晶体、LHP、LHA、TSCC和D-LTT的介电光谱。结果总结如下：1。在LHP和LHA相变温度附近存在明显的介电临界减速。结果表明：铁电相变的机制是有序-无序的。已知TSCC是第一个同时显示声子模式临界软化和弛豫模式临界减速的例子。本研究澄清了以往报道的临界松弛模态不是松弛模态，而是过阻尼软模态。在氘化LTT （D-LTT）中没有观察到临界弛豫模式，而在非氘化LTT中则没有。相变的性质不受氘化的影响。然而，位移铁电LTT表现出不同寻常的相变氘化效应；转变温度随氘比的增加而降低。当氘化比大于95%时，晶体中的铁电相变消失。

The measurement system of low-frequency dielectric spectrum has been developed. This measurement system is able to speedly provide reliable measurement data in the frequency region of 45 MHz to 15 GHz, and in the temperature region of 80 K to 450 K.The dielectric spectra of ferroelectric crystals, LHP,LHA,TSCC and D-LTT have been investigated by using the developed measurement system. The results are summarized as follows ;1.Apparent dielectric critical slowing-down are found around the phase transition temperatures of LHP and LHA.The results indicate that the mechanism of the ferroelectric phase transition is of an order-disorder.2.TSCC is known to be the first example which shows both the critical softening of phonon mode and the critical slowing-down of the ralaxational mode. The present study clarifies that the critical relaxational mode reported previously is not the relaxational mode but the over-damped soft mode.3.Critical relaxational mode is not observed in deuterated LTT (D-LTT), as in not-deuteraterd LTT.The nature of the phase transition is not affected by deuteration. However, It is found that the displacive ferroelectric LTT shows unusual deuteration effect of phase transition ; the transition temperature is lowered with increasing the ratio of deuteron. The ferroelectric phase transition disappears in the crystal with the deuteration ratio more than 95%.